The single wavelength optical detector, which detects infrared radiation of a certain wavelength, can measure the intensity of the infrared radiation of only one sensible wavelength out of the infrared radiation having a wide spectrum emitted from an object.
Accordingly, the single wavelength infrared photodetector cannot identify the temperature of an object and the distance from the infrared photodetector to the object. That is, when the infrared intensity of the sensible wavelength, which has been measured by the single wavelength infrared photodetector has a certain value, it is impossible to judge whether such infrared intensity is given because the object-to-be-detected, i.e., the infrared radiation source, is remote but has a high temperature, or such infrared intensity is given because the object-to-be-detected has a low temperature but locates near.
To judge a temperature of an object and a distance to the object, based on the infrared radiation emitted by the object, the multiwavelength infrared photodetector, which measures infrared radiation intensities of a plurality of wavelengths is used. The multiwavelength infrared photodetector can judge, based on infrared radiation intensities of the respective wavelengths having respective sensitivities, whether the temperature of an object-to-be-detected is high or low and whether the object locates remote or near.
As such multiwavelength infrared photodetector is proposed a photodetector including a plurality of infrared absorption layers of sensitivities to different wavelengths, which are stacked on a substrate (refer to, e.g., Japanese National Publication of International Patent Application No. 2002-503877, and Mani Sundaram et al., “Two-color quantum well infrared photodetector focal plane arrays”, Infrared Physics & Technology, Volume 42, pp. 301-308 (2001)).
The structure of the proposed multiwavelength infrared photodetector will be explained with reference to FIG. 19. FIG. 19 is a sectional view illustrating the structure of a double wavelength infrared photodetector which is one example of the proposed multiwavelength infrared photodetector.
As illustrated, a first electrode layer 102 is formed on a substrate 100. On the first electrode layer 102, a first infrared absorption layer 104 which is sensitive to the infrared radiation of a prescribed wavelength is formed. On the first infrared absorption layer 104, a second electrode layer 106 is formed. On the second electrode layer 106, a second infrared absorption layer 108 whose sensitivity is different from the sensitivity of the first infrared absorption layer 104 is formed. On the second infrared absorption layer 108, a third electrode layer 110 is formed.
In the region of a part of the first electrode layer 102, the third electrode layer 110, the second infrared absorption layer 108, the second electrode layer 106 and the first infrared absorption layer 104 are removed, and on the first electrode layer 102 in this region, an electrode 112 is formed. The electrode 112 is for reading photocurrent or photovoltaic power generated by absorption of radiation in the first infrared absorption layer 104.
In the region of a part of the second electrode layer 106, the third electrode layer 110 and the second infrared absorption layer 108 are removed. On the second electrode layer 106 in this region, a common electrode 114 is formed.
On the third electrode layer 110, an electrode 116 is formed. The electrode 116 is for reading photocurrent or photovoltaic power generated by absorption of radiation in the second infrared absorption layer 108.
Thus, the double wavelength infrared photodetector including the first infrared absorption layer 104 and the second infrared absorption layer 108 is constituted.
The following is another example of related art of the present invention: Japanese Laid-open Patent Publication No. 2006-058588.
By the ordinary semiconductor device manufacturing method it is difficult to discretely form substrate plane-wise a plurality of infrared absorption layers having sensitivities to different wavelengths of infrared radiation. As described above, a plurality of infrared absorption layers are stacked vertically to the substrate.
On the other hand, the electrodes for reading electric signals of absorption of infrared radiation must be formed for the respective plural infrared absorption layers. In the multiwavelength infrared photodetector as illustrated in FIG. 19, for each infrared absorption layer, it is necessary to etch off the semiconductor layers containing the electrode layer connected to the infrared absorption layer formed over the infrared absorption layer to form the electrode on the exposed electrode layer.
In such multiwavelength infrared photodetector, the electrodes must be thus formed for the respective plural infrared absorption layers, which causes demerits that the electrode forming steps are complicated, the downsizing is limited, the read circuit for reading the electric signals is complicated and large-scaled, and other demerits.